1. Field of the Invention
The present disclosure generally relates to image forming and, more particularly, to an image forming apparatus that includes a function of measuring color in a measurement image.
2. Description of the Related Art
Aspects of the quality of an image (hereinafter referred to as image quality) formed by an image forming apparatus include graininess, in-plane uniformity, character integrity, and color reproducibility (including color stability). Today, with the popularization of multicolor image forming apparatuses, it is sometimes said that the most important aspect of image quality is color reproducibility.
People have a memory relating to an expected color (especially skin color, blue sky, metals and the like) based on experience. When a color exceeds an allowable range of the memory, the color feels strange. These colors are called memory colors, and the reproducibility of those colors is often important when a photograph or the like is output.
Not only in photographic images but also in document images, increasing demands are being placed on the level of color reproducibility (including color stability) that an image forming apparatus needs to have by office users, who feel a difference between the reproduced color and that on the monitor, and graphic art users who require color reproducibility in computer graphics (CG) images.
Therefore, to satisfy the demands for color reproducibility from users, an image forming apparatus has been proposed that reads a measurement image (a patch image) formed on a sheet with a measurement unit (a color sensor) provided on a sheet conveyance path (e.g., refer to Japanese Patent Application Laid-Open No. 2004-086013). According to this image forming apparatus, a predetermined density, gradation, and tint can be reproduced by applying feedback on processing conditions, such as exposure amount and development bias, based on a reading result of the patch image by the color sensor.
However, the color measurement accuracy of the color sensor discussed in Japanese Patent Application Laid-Open No. 2004-086013 deteriorates due to factors such as variation in the output of the light source caused by changes in the environmental temperature. One way to resolve this problem is to arrange a white reference plate at a position opposite the color sensor and perform processing to correct the measurement value of the color sensor by measuring the white reference plate with the color sensor.
If a white reference plate is provided, highly accurate measurement can be performed by setting the distance from the color sensor to the white reference plate to be equal to the distance from the color sensor to the sheet when measuring the patch image. Consequently, it is desirable to arrange the white reference plate at a position where the sheet passes through during the measurement by the color sensor (hereinafter referred to as a “target position”).
However, if the white reference plate is fixed at the target position, a jam can occur due to the sheet on which the patch image is formed contacting the white reference plate. To prevent this, a white reference plate attaching/detaching mechanism needs to be provided, so that during the measurement of the white reference plate the white reference plate is moved to the target position (attachment operation), and during the measurement of the patch image on the sheet the white reference plate is moved away from the color sensor (detachment operation).
On the other hand, when the patch image is measured, light irradiated from the color sensor passes through the sheet, and is irradiated on the white reference plate. Light reflected from the white reference plate passes back through the sheet and is received by the color sensor. Since the amount of reflected light from the white reference plate varies depending on the distance from the color sensor to the white reference plate, there is the problem that if the separation position of the white reference plate is different, the measurement value of the patch image varies.